I'm pretty sure that around the time of the Afterburner kits (for non-SP GBA) came out there were a bunch of mods, like overclocking and turbo button switches, but nothing that added an extra button, since that would be near impossible because you'd need to modify the motherboard in some way to add the actual extra buttons.
I suppose you could have some kind of non-contact button (all the buttons on GBA on have a 'membrane' that when pushed down bridges a connection and however long the button is held down is how long that connection stays open) them wired in some fashion to the existing buttons and have them 'pulse', but I think getting the whole thing compact enough to fit inside a GBA/GBASP would be difficult.

I recall the thing thaddius is on about but if you really cared would it be that hard to wire an oscillating signal onto the GBA somewhere (without checking I am not sure but there should be test points you can latch on to and if not the pads/traces themselves are not too hard to play with)?
Have a look at some of the "simple" 360 controller mods (those people charging $50 for such a hack are chancers) for rapid fire as it is an almost identical idea.

Personally though if it was just for one game/genre/series I would head down the hacking patch. Most control hacks are a fairly similar in nature (find where the button IO memory location is copied to, if it is copied, then make it alternate/oscillate under certain conditions- usually if shoulder buttons and the A button are pressed or something like that).
It is for the DS touch screen but the idea is almost identical http://crackerscrap.com/ (documentation -> StarFox Command Control Hacking Tutorial ).

JK, If you want to make turbo buttons you will first need to find space on the gameboy itself. And you will need to know programming and circuting. Circute the turbo buttons to the main board and then you will need programming to programm the gameboy to identify the buttons as "rapid a/b" and then make a patch for the game ( This can only work on flashcarts) to identify the buttons. Well its hard and instead of doing this you would rather build a DS flashcart.

JK, If you want to make turbo buttons you will first need to find space on the gameboy itself. And you will need to know programming and circuting. Circute the turbo buttons to the main board and then you will need programming to programm the gameboy to identify the buttons as "rapid a/b" and then make a patch for the game ( This can only work on flashcarts) to identify the buttons. Well its hard and instead of doing this you would rather build a DS flashcart.

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Not true in the slightest. I'm pretty sure you'd just wire up buttons in such a way that when they are held, they constantly start and then stop the signal sent by pressing the button. A very common mod on most consoles, which does NOT require modification of the game in any way.

I'm pretty sure that around the time of the Afterburner kits (for non-SP GBA) came out there were a bunch of mods, like overclocking and turbo button switches, but nothing that added an extra button, since that would be near impossible because you'd need to modify the motherboard in some way to add the actual extra buttons.
I suppose you could have some kind of non-contact button (all the buttons on GBA on have a 'membrane' that when pushed down bridges a connection and however long the button is held down is how long that connection stays open) them wired in some fashion to the existing buttons and have them 'pulse', but I think getting the whole thing compact enough to fit inside a GBA/GBASP would be difficult.

Click to expand...

Yeah I was thinking about add a switch so it makes the a and b turbo then flip the switch to turn it off. And I have some 555timers to work with so I'll do some more research

You join and bump a 3 year old, mostly answered, thread for a soft sales pitch for something that basically every PC emulator will have for free? Did this thread come up especially high in the search rankings or something?